A lot of times I took over abandons with no strings attached. With the amount of open teams a lot of Commishes just want a warm body to run the team. Sometimes I liked the team/league enough to keep the a 2nd season. Sometimes I commited to a 2nd for the abandoned team, and did so, but didn't like the league for whatever reason.
My record has a lot of abandons on it, but none of the good teams I had to give away for personal reasons. Most people here know that I wont wreck the team and have NEVER abandoned a team. Everytime I've I handed the team over while I was still managing it.
Why others do one and dones I have no idea. Maybe they get bored and move around. If you want a longer commitment, ask for it. You get a 10$ bonus every 5, but that really isn't a HUGE incentive. Maybe if they made a free season, more would be inclined to stay 5 seasons or more.